Porous carbonate echinoderm or scleractinian skeletal material of marine life has a unique structure. This material has a uniformly permeable microporous structure characterized by a substantially uniform pore volume in the range from about 10 to about 90% and by a pronounced three-dimensional fenestrate structure. The microstructure of this material is somewhat similar to the cancellous structure characteristic of boney tissue or bone. Because of this unique microstructure of the porous carbonate echinoderm or scleractinian coral skeletal material of marine life these materials would appear to be useful as bone substitute material. However, the carbonate of this material, such as provided in echinoid spine calcite and Porites skeletal aragonite, do not have the desired durability for employment as bone substitutes. These materials, however, including their unique above-mentioned microporous structure, have been replicated in other materials, such as metals, which would appear to possess better physical properties from the point of strength and durability while at the same time providing the distinct unique microporous structure of the original porous carbonate coral skeletal material U.S. Pat. No. 3,890,107 discloses techniques, and products resulting therefrom, for replicating the unique microporous structure of the above-mentioned coral material including derivatives thereof.
It is also known that the aforementioned coral materials may be converted by chemical techniques employing a hydrothermal exchange reaction so as to convert the carbonate or the calcium carbonate of the coral material to hydroxyapatite while at the same time retaining the unique microstructure of the coral material. U.S. Pat. No. 3,929,971 discloses a hydrothermal exchange reaction for converting the porous carbonate skeletal material of marine life into a phosphate or hydroxyapatite skeletal material possessing the same microstructure as the carbonate skeletal material. These synthetic hydroxyapatite materials have been produced commercially and are available from Interpore International Inc., Irvine, Calif., under the tradename Interpore-200, which is derived from certain coral of the genus Porites which have an average pore diameter of about 200 .mu.m and under the tradename Interpore-500 derived from certain members of the family Goniopora,, which have pore diameters of about 500 .mu.m.
These special Interpore hydroxyapatite materials have also been identified as replamineform hydroxyapaptite and coralline hydroxyapatite. Interpore-200 and Interpore-500 have been found to be useful as bone substitute materials. More information concerning these materials is to be found in the article by Eugene White and Edwin C. Shors entitled "Biomaterial Aspects of Interpore-200 Porous Hydroxyapatite", which appeared in Dental clinics of North America, Vol. 30, No. 1, January 1986, pp. 49-67.
In addition to the above-described materials which have the unique microstructure of porous skeletal coral material, other materials have been proposed as bone substitute materials, see U.S. Pat. Nos. 4,097,935, 4,195,366, 4,308,064 and 4,314,380. For the most part, however, these other bone substitute materials which do not possess the unique structure of coral material which is possessed by Interpore-200 and Interpore-500, have not been completely satisfactory.
Despite the fact that calcium phosphates have been well investigated, see the publication entitled Bioceramics of Calcium Phosphate, particularly Chapter 1 of F. C. M. Driessens entitled "Formation and Stability of Calcium Phosphates in Relation to the Phase Composition of the Mineral in Calcified Tissues", and Chapter 5 by Klaas deGroot entitled "Ceramics of Calcium Phosphates: Preparation and Properties", other calcium phosphate materials which possess the advantages and the unique coral-derived microporous structure of Interpore-200 and Interpore-500 have not yet been satisfactorily produced.
The disclosures of the above-identified patents and publications are herein incorporated and made part of this disclosure.
The physical properties of the apatite bone substitute materials which possess the unique microstructure of skeletal material, such as Interpore-200 and Interpore-500, although satisfactory, do not provide for all the needs of surgeons employing the same as bone replacements and bone implant materials. For example, some surgeons would prefer a similar material but made up of a more readily absorbable or resorbable material, such as a material which would be absorbed by the body or would disintegrate within about six months to two years. Other surgeons would prefer to employ a similar such material which is more refractory, lasts about ten years, more or less, or be substantially permanent. The presently available materials, such as Interpore-200 and Interpore-500, possess properties somewhat intermediate and are rather fixed since these materials are comprised substantially only of hydroxyapatite.
It is an object of this invention to provide bone substitute materials and a method for their manufacture derived from hydroxyapatite or other calcium phosphate bone substitute material having the unique microstructure of the porous carbonate echinoderm scleractinian coral skeletal material of marine life.
It is another object of this invention to provide bone substitute materials derived from hydroxyapatite material or other calcium phosphate bone substitute material which has the unique microstructure of the porous carbonate echinoderm or scleractinian coral skeletal material of marine life or the cancellous structure characteristic of boney tissue or bone but which is chemically different from hydroxapatite or the material from which it is derived but yet possessing substantially the same microstructure of the material from which it is derived and which is relatively more or less readily absorbable by the body.
How these and other objects of the invention are achieved will become apparent in the light of the accompanying disclosure made with reference to the accompanying drawing which illustrates a portion of the phase diagram of the system CaO-P.sub.2 O.sub.5.